The present invention relates generally to disc drive data storage systems. More particularly, the present invention relates to canceling resonance disturbances in disc drive systems.
In a computer disc drive, data is stored on a disc in concentric tracks. In disc drives with relatively high track densities, a servo feedback loop is used to maintain a head over the desired track during read and write operations. This is accomplished utilizing prerecorded servo information either on a dedicated servo disc or on angularly spaced sectors that are interspersed among the data on the disc. During track following, the servo information sensed by the head is demodulated to generate a position error signal (PES), which provides an indication of the position error of the head away from the track center. The PES is then converted into an actuator control signal, which is fed back to an actuator to position the head.
In general, there are two forms of head positioning errors: repeatable and non-repeatable. Repeatable errors, which are generally caused by mechanical irregularities in the structure of the disc drive or errors introduced when writing the servo tracks, can be predicted and therefore theoretically can be cancelled out as they occur. In general, these repeatable runout errors (RRO) are removed by introducing a compensation signal into the servo loop that cancels the repeatable positioning error. Techniques for generating such compensation signals are generally referred to as feedforward cancellation.
Nonrepeatable runout (NRRO) can account for 30-40% of total runout. Feedforward cancellation is much more difficult to implement for nonrepeatable runout errors. Nonrepeatable runout disturbances can arise as a result of a variety of factors and the characteristics of the disturbance often vary depending on the source of the disturbance. Disc flutter disturbances have high energy concentrated at specific frequencies (typically between approximately 500 and 1000 Hz). On the other hand, suspension windage-induced disturbances occur at higher frequencies and have wider bandwidths. Also, unlike repeatable runout errors, nonrepeatable runout errors do not occur with predictable magnitudes, frequencies or phases. In fact, the magnitude, frequency and phase of a nonrepeatable runout error signal generally change with each rotation of the disc.
One system, discussed by Yu in U.S. Pat. No. 5,072,318, attempts to cancel a nonrepeatable runout error by adaptively changing a cancellation signal at each sector of the disc. Specifically, Yu uses a least squares algorithm to adaptively change the frequency, phase, and amplitude of the nonrepeatable runout cancellation signal at each sector.
Because Yu uses a least squares algorithm to determine the amplitude, frequency and phase of the cancellation signal, it takes time for the Yu algorithm to converge on the proper amplitude, frequency and phase. This delay in converging on the amplitude, frequency and phase of the nonrepeatable runout error causes the produced cancellation signal to be less than ideal. Also, because the feedforward scheme described by Yu uses an algorithm that includes frequency search, it results in a system that is adaptive, nonlinear and time-varying. Thus, the system is not scalable and is prone to divergence for larger learning rates and for certain disturbance frequencies.
The present invention provides a solution to this and other problems and offers other advantages over the prior art.
The present invention relates to canceling resonance disturbances in disc drive systems.
One embodiment of the present invention is directed to a method of canceling nonrepeatable runout position error. Pursuant to the method, a frequency of a specified nonrepeatable runout disturbance is determined. A position error signal indicative of a position of a read/write head relative to a track is received. A signal is generated based upon the frequency of the specified nonrepeatable runout disturbance and the received position error signal. The generated signal is applied to the servo loop. Applying the generated signal to the servo loop cancels a component of the position error signal arising from the specified nonrepeatable runout disturbance.
In one embodiment of the above method, applying the generated signal to the servo loop produces a notch in an error sensitivity function relating the position error signal to a general nonrepeatable runout disturbance, the notch being centered at the frequency of the specified nonrepeatable runout disturbance.
In a further embodiment of the above method, the signal is generated according to the equation:             A      ⁢              (        z        )              =                  u        PES            =                                                  z              2                        ⁡                          [                                                                    μ                    o                                    α                                ⁢                                  cos                  ⁢                                      (                    φ                    )                                                              ]                                -                      z            ⁡                          [                                                                    μ                    o                                    α                                ⁢                                  cos                  ⁢                                      (                                          φ                      +                                                                        ω                          o                                                ⁢                        T                                                              )                                                              ]                                                                          z              2                        η                    -                      z            ⁡                          [                              2                ⁢                                  cos                  ⁢                                      (                                                                  ω                        o                                            ⁢                      T                                        )                                                              ]                                +          η                      ,
where u is the generated signal, PES is the position error signal, z is the z transform, xcfx890 is the frequency of the specified nonrepeatable runout disturbance, xcexc0 is the width of the notch, xcex7 is the depth of the notch, xcex1 is a magnitude of a nominal closed loop gain of the servo loop at the frequency xcfx890, xcfx86 is a nominal closed loop phase response of the servo loop at the frequency xcfx890, and T is a sampling period.
Another embodiment of the present invention is directed to a disc drive having an actuator, a servo controller and a filter. The actuator operates in a servo loop to position a read/write head relative to a track on a disc surface. The servo controller receives a position error signal indicative of a position of the read/write head relative to a track on the disc surface and provides a control signal to the actuator based upon the position error signal. The filter receives the position error signal and provides a cancellation signal to the servo loop based upon the received position error signal and a frequency of a predetermined nonrepeatable runout disturbance. The compensation signal is adapted to cancel a component of the position error signal arising from the predetermined nonrepeatable runout disturbance.
In one embodiment of the above-described disc drive, the cancellation signal is adapted to produce a notch in an error sensitivity function relating the position error signal to a general nonrepeatable runout disturbance. The notch is centered at the frequency of the predetermined nonrepeatable runout disturbance.
In an illustrative embodiment of the above-described disc drive, the cancellation signal is calculated according to the equation:             A      ⁢              (        z        )              =                  u        PES            =                                                  z              2                        ⁡                          [                                                                    μ                    o                                    α                                ⁢                                  cos                  ⁢                                      (                    φ                    )                                                              ]                                -                      z            ⁡                          [                                                                    μ                    o                                    α                                ⁢                                  cos                  ⁢                                      (                                          φ                      +                                                                        ω                          o                                                ⁢                        T                                                              )                                                              ]                                                                          z              2                        η                    -                      z            ⁡                          [                              2                ⁢                                  cos                  ⁢                                      (                                                                  ω                        o                                            ⁢                      T                                        )                                                              ]                                +          η                      ,
where u is the cancellation signal, PES is the position error signal, z is the z transform, xcfx890 is the frequency of the predetermined nonrepeatable runout disturbance, xcexc0 is the width of the notch, xcex7 is the depth of the notch, xcex1 is a magnitude of a nominal closed loop gain of the servo loop at the frequency xcfx890, xcfx86 is a nominal closed loop phase response of the servo loop at the frequency xcfx890, and T is a sampling period.